As reduction of the greenhouse gas carbon dioxide (CO2) is becoming more and more imperative, a practical way of improving the efficiency of the automobile engine is urgently needed.
It is well known that the efficiency of the Spark-Ignition (SI) engine is directly dependent on engine load. Most modern SI engines run at a fixed, preferably stoichiometric, Air/Fuel Ratio—hereinafter referred to as AFR. Output regulation is accomplished by throttling the intake duct, thereby reducing the mass flow of air, or combustible mixture. Thus, the SI engine is most efficient at full load, where throttling is absent. Its efficiency is lower at part load operation, mainly due to the following two factors of influence, working in co-operation:    1. Increase in pumping losses with throttling.    2. Reduction of the cylinder pressure, at the end of the compression stroke, hereinafter referred to as compression pressure, or Pc.
Automobile engines operate at part load for most of the time, which significantly impairs vehicle fuel economy and increases CO2 emissions. Hence, improving part-load efficiency will have the largest impact on the overall fuel economy of the automotive engine.
One possible approach is to act upon the first of the two aforementioned factors of influence, i.e. throttling. Internal mixture formation systems (direct injection) have been developed, which allow the SI engine to run unthrottled, at certain speeds and loads. Engine power is controlled by varying the air-fuel ratio, much like in a Diesel engine. Unthrottled engines inherently operate over a wide AFR range, from very lean at idle and light load, to near stoichiometric at full load. Airflow is essentially constant and output regulation is achieved by modifying the fuel flow rate, and subsequently the AFR.
Emissions control technologies for the stoichiometric-burn SI engine have reached extremely high efficiency, through many decades of refinement. The mainstream emissions reduction strategy relies on Three-Way Catalytic (TWC) exhaust gas aftertreatment, which requires running with a stoichiometric AFR throughout the entire speed/load range of the engine. The stoichiometric SI engine with TWC aftertreatment has been honed into a very efficient, reliable and cost-effective solution.
In contrast, the exhaust aftertreatment techniques for the lean mixtures used by the unthrottled SI engine are relatively new and still far from the efficiency and cost effectiveness of the TWC.
The above brief overview will make it apparent that SI engine efficiency improvement by eliminating the throttle is difficult and expensive.
Manipulating the second factor of influence, i.e. compression pressure, is a well-known theoretical path to increasing SI engine efficiency, but a practical solution has yet to be developed.
Although usually referred to as Variable Compression Ratio (VCR), this approach would perhaps be more aptly called Constant Compression [Pressure] Engine, as the aim is to maintain a constant compression pressure, Pc, over the entire operating domain of the engine.
Throttling reduces intake manifold pressure, thereby reducing mass airflow to the engine. Thus, a higher degree of throttling may appear equivalent to utilizing a smaller displacement engine. The critical difference, however, is that Pc is also reduced with throttling. Indeed, if a constant Pc could be maintained, irrespective of intake manifold pressure, part-load operation would be much more similar to running a smaller engine at full load and thereby at its peak efficiency. Admittedly, the increased pumping losses will somewhat offset the theoretically constant efficiency.
Evidently, holding Pc constant means altering the geometrical compression ratio, hereinafter referred to as CR, as clearly illustrated by the equation:Pc=Pa·CRnc Where: Pa is the cylinder pressure at the beginning of the compression stroke, and nc is the polytropic coefficient of the compression process.
A most important advantage of this approach is that it fully exploits mature and cost-effective fuel metering and exhaust aftertreatment technologies, i.e. port fuel injection and TWC, respectively.
It should also be noted that an attractive method to rise engine specific output is to increase intake manifold pressure, at high load, above atmospheric. The technique, well known to those skilled in the art, is referred to as supercharging and is accomplished by using some type of air compressor, or charger. One widely used arrangement utilizes a centrifugal air compressor, driven by an exhaust-gas turbine. The aggregate turbine-compressor device is called a turbocharger and its use on an engine is often referred to as turbocharging.
The higher manifold pressure, or boost, augments mass airflow, subsequently increasing the specific output of the engine. The main limiting factor is the onset of abnormal combustion, i.e. detonation, or knock, caused by the higher compression pressure of the supercharged engine. Thus, the geometrical Compression Ratio, CR, of a supercharged engine must be lower than in a similar, but normally aspirated, powerplant. That further reduces the part-load efficiency of the supercharged engine.
The two main approaches used in the prior art to control the geometric compression ratio are:                a) Altering the piston Top Dead Centre (TDC) position in respect to the cylinder head.        b) Modifying the combustion chamber volume.        
The first path relies on modified pistons or crank mechanisms, or even on cylinder heads moveable in respect to the engine block. While a few experimental engines based on this first strategy do exist (SAAB, MCE-5/Peugeot), the complexity of the solutions makes those engines difficult to mass produce at a competitive cost.
The second approach present in the prior art is modifying the geometrical compression ratio by creating a variable volume combustion chamber, or a sub-chamber within the engine combustion chamber. The volume-control device is usually a sliding piston, driven by one of many possible actuator means.
Study of the prior art reveals a number of paper solutions, all of which pose significant practical obstacles to a functionally viable implementation.
Referring now to said second approach, while the idea is, in principle, sound—and essentially obvious to one skilled in the art, there are several serious practical impediments associated with the prior art concepts, as follows:
The combustion pressure of a typical SI engine is in the 100 bar range, which imparts kN level forces to the sliding piston, in a high-gradient, pulsating manner. The high-pressure pulses alternate with low-pressure ones, occurring during the intake strokes of the engine. The rapidly fluctuating cylinder pressure will cause the sliding piston to oscillate, thereby uncontrollably altering the effective compression ratio of the engine. Rigid and bi-directional locking means must be included in the piston actuation mechanism, to prevent the sliding piston from oscillating.
If a cam is used to directly drive the sliding piston, the high-pressure forces acting on the piston also create a substantial frictional load at the point of contact between piston and cam.
Generally, prior art work does not provide an explicit solution for keeping the sliding piston and its actuator in permanent contact. An exception is U.S. Pat. No. 5,195,469 (Syed), wherein the piston is still unidirectionally driven by a cam, but a spring is used to maintain piston-to-cam contact. However, considering the highly dynamic forces involved, a spring-loaded piston is very likely to temporarily loose contact with the actuation cam and bounce.
Moreover, during acceleration, the automotive engine often rapidly transitions from idle, i.e. highest desired CR, to full load, i.e. lowest desired CR. The transition time may be as short as 100 ms and the volume-control device must be equally fast. If the volume-control device motion lags throttle opening, Pc will reach dangerously high levels, causing violent detonation, which can quickly destroy the engine. That precludes the use of most screw type actuators proposed in prior art.
For the same reason, when a sliding piston is used, it is desirable for its stroke to be as short as possible, which means that the piston area must be as large as possible, within the load constraints on the actuation mechanism. However, the larger the sliding piston, the less room is left, in the combustion chamber, for the intake and exhaust valves.
That is especially true with modern automotive SI engines, optimized for high output at full load, which often utilize multiple intake and exhaust valves, per cylinder.
Furthermore, the valvetrain actuation mechanism, spark plugs, and possibly fuel injectors, occupy most of the space available in the cylinder head, above the combustion chamber. It is difficult to see how a sliding piston and its actuation mechanism could fit in that same space.
On the same note, some prior art arrangements show the spark plug mounted onto the sliding piston. This setup exposes the spark plug to the operating environment existing on the backside of the piston, i.e. inside the engine valve cover. Not only is that environment already rich in oil vapor, but also additional oil is highly desirable, for cooling the slider. Oil is electrically conductive, effectively short-circuiting the spark plug.
Accordingly, the main objective of this invention is to provide a practical Constant Compression Pressure Engine solution.